CAPT OF A DC9 FAILED TO HOLD SHORT OF AN ACTIVE RWY ON WHICH A B757 WAS ON TKOF ROLL RESULTING IN THE CAPT REALIZING HIS MISTAKE AND CONTINUING ON QUICKLY TO CLR THE RWY AND THE B757 CONTINUING TKOF.

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AFTER THE CAPT'S LNDG ON RWY 21L AT DTW; THE TWR CTLR INSTRUCTED US TO HOLD SHORT OF RWY 21C AT TXWY F. I ACKNOWLEDGED THE HOLD SHORT CLRNC WITH OUR CALL SIGN; AND THE CAPT AND I DISCUSSED THE HOLD SHORT CLRNC; AS WELL AS OUR PROBABLE TAXI RTE TO THE GATE. I WAS IN THE PROCESS OF COMPLETING THE 'AFTER LNDG' CHKLIST; AND I WAS OBSERVING THE DEPARTING ACFT LINED UP FOR RWY 21C WHEN I LOOKED UP AND SAW OUR ACFT APCHING THE HOLD SHORT LINE IN QUESTION. I SHOUTED AT THE CAPT TO 'HOLD SHORT!' AND HE AGGRESSIVELY APPLIED THE BRAKES; BUT THE NOSE OF OUR ACFT HAD PASSED ACROSS THE HOLD SHORT LINE FOR RWY 21C. RAPIDLY ASSESSING THE SIT WITH THE DEPARTING ACFT; THE CAPT ELECTED TO CLR RWY 21C BY EXPEDITIOUSLY TAXIING ACROSS THE RWY AT TXWY F. WHILE WE WERE TAXIING ACROSS THE RWY; AN ACR Y ACFT HAD BEEN CLRED FOR TKOF ON RWY 21C AND HAD BEGUN HIS TKOF ROLL. THE ACR Y ACFT COMPLETED A NORMAL TKOF; AND THE TWR CTLR DID NOT REALIZE WE HAD INADVERTENTLY CROSSED THE RWY UNTIL AFTER THE EVENT WAS OVER. I DO NOT KNOW WHAT OUR LATERAL SEPARATION WAS FROM ACR Y; BUT I DO KNOW THAT WE HAD COMPLETELY CROSSED THE RWY WHILE THAT ACFT WAS EARLY IN HIS TKOF ROLL. LATER WHEN ASKED WHY HE DID NOT HOLD SHORT; THE CAPT EXPLAINED THAT HE HAD HEARD THE TWR CTLR CLR AN ACFT 'INTO POS AND HOLD; TFC XING DOWNFIELD.' THE CAPT THOUGHT WE HAD BEEN CLRED TO CROSS RWY 21C; WHEN IN FACT WE HAD NOT. HINDSIGHT TOLD HIM THAT THE CLRNC 'INTO POS AND HOLD' ETC; WAS FOR THE ACFT BEHIND ACR Y ACFT PREVIOUSLY DISCUSSED. ANALYSIS: TFC ON RWY 21L AND RWY 21C WAS BEING CTLED ON 1 TWR FREQ; 118.4; AND THE FREQ WAS VERY BUSY. POSSIBLY THE CAPT HEARD SOMETHING THAT HE MISCONSTRUED AS A CLRNC FOR US TO CROSS. AT SOME ARPTS YOU SWITCH TO GND CTL FREQ AFTER LNDG WHILE HOLDING SHORT OF ACTIVE RWYS. THIS IS VERY SIMPLY A HUMAN ERROR INCIDENT. THE CAPT TOOK ACTION BASED ON A CLRNC HE DID NOT HAVE. THE FO AFTER DISCUSSION WITH THE CAPT; WAS CONFIDENT THE CAPT UNDERSTOOD HE WAS TO HOLD SHORT OF RWY 21C. PERHAPS THE LESSON LEARNED SHOULD BE AS A HOLD SHORT POINT IS APCHED; BOTH PLTS SHOULD ONCE AGAIN CONFIRM THAT THEY ARE TO HOLD SHORT AT THE APPROPRIATE POINT; EVEN TO THE EXCLUSION OF ALL OTHER DUTIES.